Process for producing cold-rolled steel plate high in the cold-formability

ABSTRACT

COLD-ROLLED STEEL HAVING EXCELLENT COLD-FORMABILITY IS MADE BY POURING INTO A MOLD A MOLTEN STEEL HAVING A COMPOSITION OF C$0.07 WT. PERCENT, 0.04 TO 0.20 WT. PERCENT MN, 0.004 TO 0.020 WT. PERCENT S, THE RATIO OF MN TO S BEING AT LEAST 7, NOT MORE THAN 0.0030% N, AND THE BALANCE IRON AND IMPURITIES. THE MOLTEN STEEL IS ALLOWED TO RIM IN THE MOLD FOR A TIME INTERVAL. THEN THE CORE OF THE MOLTEN STEEL IS KILLED BY ADDING AL TO THE MOLTEN STEEL AFTER THE TIME INTERVAL, SUCH THAT THE ACID-SOLUBLE AL IN THE REMAINING MOLTEN STEEL IS MORE THAN 0.010% TO   OBTAIN AN INGOT. THE INGOT IS THEN SUBJECTED TO HOTROLLING, COLD-ROLLING RECRYSTALLIZING AND ANNEALING.

June 6, 1972 MINEO SHIMIZU ETA!- 3,553,016

PROCESS FOR PRODUCING COLD-ROLLED STEEL PLATE'HIGH I IN THECQLD-FORMABILITY Fued March 5, 1969 3 Shuts-Shut 1 O I.8 O

O l.6- O

O o O O 0 1.4 o O O o o lllll'lllllllllllllll INVENIORS M me a Shim/2U Hl'ros/u' Takec/u' Hiroyuk/ Kaj/o/ra M/noru Kawaharada BY,

UQMMWNXMM PM ATTORNEYS June 6, 1972 MINEO SHIMIZU ETA!- 3,663,016

PROCESS FOR PRODUCING COLD-ROLLED STEEL PLATE HIGH IN THECOLD-FORMABILITY Flled Maren 5, 1969 3 Sheets-Shoot 2 FIG. 2

22 0E1 on I I I l I l I 2 4 6 8 0.0I0 l2 l4 l6 l8 0.020 22 24 26 280.030

INVENTORS M in ac Sh I'm in Hirosh/ Ta/rechi H/rvyu/r/ Kajio/ra M/rwwawa/Iamda ATTORNEYS J1me 1972 MINEO SHIMIZU ETAL 3,668,016

PROCESS FOR rnonucme com-momma sum:- PLATE men 4 IN THE cow-romsxm'mrFlled March 5. 1969 .3 Shots-53! s INVENTORS MINEO SHIMIZ'U HIROSHITAKECHI HIROYUKI KA M-INORU KAWAl-IARADA ATTORNEYS United States PatentOfiice 3,668,016 Patented June 6, 1972 3,668,015 PROCESS FOR PRODUCINGCOLD-ROLLED STEEL PLATE HIGH IN THE COLD-FORMABILITY Mineo Shirnizu,Hiroshi Takechi, Hiroyuki Kajioka, and Minoru Kawaharada, Kitakyushn,Japan, assignors to Nippon Steel Corporation, Tokyo, Japan Filed Mar. 3,1969, Ser. No. 803,669 Claims priority, application Japan, Mar. 2, 1968,43/ 13,459 Int. Cl. 322d 25/06 US. Cl. 148-2 1 Claim ABSTRACT OF THEDISCLQSURE Cold-rolled steel having excellent cold-formability is madeby pouring into a mold a molten steel having a composition of C0.07 wt.percent, 0.04 to 0.20 wt. percent Mn, 0.004 to 0.020 wt. percent S, theratio of Mn to S being at least 7, not more than 0.0030% N, and thebalance iron and impurities. The molten steel is allowed to rim in themold for a time interval. Then the core of the molten steel is killed byadding Al to the molten steel after the time interval, such that theacid-soluble Al in the remaining molten steel is more than 0.010% toobtain an ingot. The ingot is then subjected to hotrolling, cold-rollingrecrystallizing and annealing.

This invention relates to a cold-rolled steel plate having few surfaceflaws and excellent cold-formability and a process for producing thesame.

In the case of forming a cold-rolled steel plate by a pressingoperation, it is required that the steel plate should be nonageable andshould have good deep-drawability and extrudability depending on theuse.

If a cold-rolled steel plate shipped from a rolling mill after beingtempered and rolled ages before it is subjected to a press working,there will be produced a nonuniform irregular pattern called a stretcherstrain when it is pressed, which will remarkably impair the appearanceof the formed product and will reduce the press-workability. Therefore,it is requisite that the material should be nonageable.

The main cause of such ageing phenomenon is considered to be Nsolid-dissolved in the steel and can be prevented by adding an elementwhich will combine with N. Among such additive elements, cheap Al isused in most cases. An aluminum-killed steel containing more than 0.02%acid-soluble Al (which shall be referred to as sol. Al hereinafter) istypical thereof.

However, in the case of producing such steel plate, if required Al isadded into a ladle or into a mold when pouring a molten steel, A1 willmainly gather near the skin of an ingot, whereby not only the surfaceproperties of the ingot will be deteriorated, but also the surface flawsof the rolled product will be produced and the yield of the product willbe remarkably reduced.

As a measure of preventing such surface flaws it is already known to addAl immediately or several minutes after a rimmed steel is poured into amold. (The thus produced steel shall be called a rim-stabilized steelhere.) In this ingotting method, however, it is necessary to regulatethe contents of C and Mn and the like from the necessity of causing astrong rimming action to take place at the time of pouring a moltensteel, but on the other hand, the cold-formability (the deep-drawabilityand extrudability here) of the product must be deteriorated by thisregulation. Therefore, this contradiction presents a serious problem inthe industry.

The present invention seeks to solve the above described problem and hasfor an object to provide a rimstabilized steel which has favorablesurface properties and excellent cold-forrnability.

Other objects of the present invention will be clear from the followingdescription and accompanying drawings.

In the accompanying drawings:

FIG. 1 shows influences of the Mn content on the '1 value of the finalproduct when only the Mn content was varied; I

FIG. 2 shows influences of the absolute values and combination of the Mnand S contents on the F and n values of the final product when both theMn and S contents were varied;

. FIG. 3 is a photograph of a corner sample of an ingot made by aconventional method and having an Mn content of 0.30%;

FIG. 4 is a photograph of a corner sample of an ingot according to thepresent invention.

The present invention shall be described more particularly in thefollowing.

In producing a rim-stabilized steel, according to an ordinary ingottingmethod wherein Al is added immediately or several minutes after a moltensteel to be made a rimmed steel is poured, it is possible to prevent A10 from mainly gathering near the skin of an ingot and the surface defectfrom being caused by A1 0 In this case the thickness and soundness ofthe rim layer coagulated before Al is added are important in relation tothe surface flaws. It is considered that a range of 0.07 to 0.10% C isdesirable to obtain a sound rim layer.

If the above described ingotting method is applied to a molten steel ofless than 0.07% C, the rimming action is weak and there remain manysmall bubbles as shown in the photograph (steel with 0.050% C, 0.30% Mnand 0.018% S) in FIG. 3, whereby the soundness of the rim layer isremarkably reduced. The bubbles distributed near the ingot skin aresubjected to an oxidation during the soaking heating and cannot bepressed into contact, causing thereby the formation of surface flaws ofthe rolled product.

However, from the view point of the formability of the final product, 0content should be as low as possible and is required to be below 0.07%

In order to solve such contradiction, the present inventors havediscovered that, when the C content is reduced, only the Mn value in theladle is related with the removal of the generated bubbles and that,when the Mn content is held to a low value which is not conceivable inthe usual conventional cold-rolled steel plate, that is, less than 0.2%,the bubbles near the skin of the ingot (steel with 0.052% C, 0.17% Mnand 0.016% S) vanish as shown in the photograph in FIG. 4, thus theproblem of the surface flaws in the product may be solved. I

Then, the influence of the reduction of the ladle Mu value to less than0.20% on the formability of the product. becomes an important problem.

FIG. 1 shows the relation between the F value and the Mn content of aproduct manufactured from an ingot of rim-stabilized steel of 0.006% S,said steel ingot being prepared by melting so that the Mn contentthereof may be varied, but other chemical components not intentionallyvaried, by subjecting said ingot to following steps: blooming,hot-rolling, cold-rolling at a reduction rate of 70% and torecrystallization annealing at 700 C. for 4 hours.

The F value, which is also called a plastic strain ratio, is amechanical characteristic represented by the ratio of the width strainto the thickness strain of a tension test piece and is known to show avery favorable correlation with the deep-drawability of a metal plate.The

F value is a mean value in each direction in the plane of the plate.

The larger the F value, the higher the deep-drawability. It has a valueof about 1.3 in an ordinary rimmed steel or about 1.6 in a higheraluminum-killed steel.

As is clear from FIG. 1, there is recognized a definite relation betweenthe F value and Mn content. When the content is about 0.10%, the F valuewill be the maximum.

As described later, unless a sufiicient amount of Mn is contained inrelation to S, the material will break or crack due to the phenomenon ofred-hot brittleness during the hot-rolling.

The F value of the materials on the low Mn side in FIG. 1 is thatconverted from a measured value for a final product obtained from amaterial partly cracked in hot-rolling as a mother material, saidmeasured value being obtained by quantitatively measuring the crystalsurface parallel with the plate plane of the final product by X-rays.Because the sample to be used for the X-ray measurement is large eventhough it is smaller than that needed for a mechanical test, even a.partly cracked material can be used as a sample, while avoiding thecracked part. Further, as is described in detail, for instance, in theJapan Metal Society Journal, vol. 29, No. 4, 1965, the relation betweenthe value measured with X-rays and the F value measured from themechanical test is very close. In such ordinary steel plates, as forexample, the current Japanese Industrial Standard G 3310 (1965) SPC 3(steel plates to be deep-drawn), the Mn content is defined to be 0.25 to0.45%. However, from the results in FIG. 1, it. can be said to beadvantageous to the F value to further reduce the Mn content.

However, the present inventors have simultaneously discovered that the Fvalue is greatly influenced not only by the Mn content but also by the Scontent. That is to say, FIG. 2 shows the relationships of the F valueand 11 value with the Mn and S contents, when treating in the samemanner as in the case of the material of FIG. 1, 57 rim-stabilized steelingots obtained by various combinations within ranges of 0.02 to 0.40%Mn and 0.003 to 0.030% S. It is clearly recognized from this graph thatthe F value and n value are influenced by both Mn and S. In the graph,the F value and the n value are represented respectively by round andsquare marks on the same material. However, in this graph, the positionof the round mark indicates the right position. The square mark is drawnto show the n value is placed beside the corresponding round mark. Theround mark having no square mark therebeside represents that thematerial was broken due to red-hot brittleness and the n value could notbe measured.

The n value so called here is also called a work hardening index and isknown to have a good relation with the extrudability of the metal plate.The n value corresponds to the index n when the true stress (a)strain(e) curve obtained by the tension test of the material is approximatelya'=Ce (wherein C is a constant) and is generally determined by theaverage gradient of the log. c'10g. c curve in the strain range of toThe larger this value, the higher the extrudability. It is about 0.22 inan ordinary rimmed steel or aluminum-killed steel.

As is described later, the steel of the present invention is made sothat the sol. A1 contained in the steel may be more than 0.01%. However,this is about the same as in an ordinary aluminum-killed steel and issuflicient to make the steel nonageable.

However, in an ordinary aluminum-killed steel, when the acid-soluble N(sol. N) contained in the steel is about the same as in the steel of thepresent invention, that is, 20 to p.p.m., it is very difiicult to obtainan F value of more than 1.5. On the contrary, it is a great feature ofthe present invention that a favorable F value reaching 2.0 can beobtained by reducing the Mn content and balancing Mn and S.

From the results shown in FIG. 2 it is concluded that for the Mn contenta range of 0.04 to 0.20% (preferably 0.08 to 0.18%) and for the Scontent a range of 0.004 to 0.020% (preferably 0.008 to 0.018%) shouldbe selected, if an F value exceeding 1.6 which is an approximate averagelevel of the F value of an aluminumkilled steel, that is, a typical highgrade deep-drawable steel plate, and an n value of more than 0.22, whichis an approximate average level of the n value of the same, are set astargets to be attained. When Mn and S were contained in these ranges, anF value of 1.7 to 2.2 and n value of 0.26 to 0.34 of the product wereobtained. At present, conventional steels of the same kind contain Mnand S in the ranges of about 0.30 to 0.40% and 0.010 to 0.025%respectively and have an F value of 1.4 to 1.8 and an 11 value of 0.20to 0.25, which clearly demonstrates the superiority of the presentmethod, when compared with the above-mentioned values of the steel ofthe present invention.

On the other hand, in metallurgy there is phenomenon called a red-hotembrittlement, in which an embrittlement of a steel is caused when it ishot-rolled. This is considered to be caused by the fact that S in thesteel is reticulately deposited as FeS around an initial crystal. As acountermeasure for preventing this phenomenon there has been widelyadopted a practice of adding Mn in an amount matching S.

Because of the segregation of S in an ingot there has been heretoforeempirically carried out the steps of adding Mn in an amount such thatthe ratio of Mn to S; 10 in the case of a rimmed steel. However, in thecase of a rim-stabilized steel, the segregation of S is so little and 0content in the steel is so low that the value can be lower than in arimmed steel.

It is found also from the results in FIG. 2 that, when the ratio of Mnto S is at least 7 the material is not broken.

In collective consideration of the above-mentioned, it is evident thatthe Mn and S contents for obtaining sound cold-rolled steel plates highin the F and n values are to be found in the range hatched with diagonallines in the graph.

The range hatched with diagonal lines in FIG. 2 shall be called thedesignated range of FIG. 2 hereinafter.

C content has an influence on the formability, as abovementioned, andshould be as low as possible. Therefore, the C content as a ladlecomposition is made 0.07%. However, in consideration of the economy ofthe steel producing operation and the surface quality of the product,the C content as a ladle composition is made more than 0.03%. In orderto improve the workability of rimmed steel, such steel has beendecarburized by an open coil annealing system. But, it is a feature ofthe method of the present invention that, without subjecting the steelof the present invention to such a decarburization treatment, a veryhigh press-formability can be obtained.

Further, it is to be noted that a conventional aluminumkilled steelusually contains more than 0.020% acid-soluble Al. This is, of course,an amount too large for the putrhprge of making the steel platenonageable by combining W1 However, this amount is based on the idea ofsimultaneously elevating the F value by forming AlN in an op timumamount. Therefore, in the case of the present invention in which the Fvalue is increased by the balance of Mn and S, and Al is added only forthe purpose of obtaining the nonageability of steel, the content of morethan 0.010% acid-soluble Al in the product is sufiicient for the saidpurpose.

A method of carrying out the present invention shall be described in thefollowing.

A molten steel made in a converter or open-hearth furnace and having aladle composition of C 0.07% and Mn and S within the designated range ofFIG. 2 is toppoured or bottom-poured so as to be a rimmed steel. Inpouring it, some shot Al is used in response to the state of the rimmingaction. After a desired coagulated layer (rim layer) is obtained, Al isadded into the molten steel in the mold so that the sol. Al in theremaining molten steel may be more than 0.02% and the molten steel iscoagulated. A cold-rolled steel plate is made from it through respectiveblooming, hot-rolling, cold-rolling and annealing steps by aconventional process. That is to say, it is hot-rolled at a temperatureabove the Ar point, is coldrolled at a reduction rate in a range of 50to 90% and is then annealed at a temperature above the recrystallizingtemperature.

In the case of manufacturing the steel of the present invention Al isadded after a desired rim layer has been obtained when making a rimmedsteel from the molten steel, which prevents the formation of any surfacefiaw to be caused by A1 partially gathering near the ingot skin by thesound rim layer which is first obtained by making Mn 0.20% as describedabove.

The amount of Al to be added difi'ers depending on the property of themolten steel, the pouring condition and the time used for adding Al.However, in case sol. Al is 0.02%, it is necessary to add more than 0.7kg./ton of Al. The time of adding Al is somewhat different depending onsuch pouring method as top-pouring or bottom-pouring and on the pouringrate but is generally determined by taking into consideration both thethickness of the rim layer required when rolling an ingot and therefloating of A1 0 series impurities.

As to the amount and time of adding Al they are not particularlyspecified in the present invention, because they are strongly influencedby Working conditions.

The thus produced product contains C in the range of 0.06 to 0.02% andMn and S in the ranges substantially not varied, but still such that theratio of Mn to S 57, that is, in the designated range of FIG. 2.Therefore, the object is attained thereby. Even if C is made less than0.02%, the product cannot be expected to improve in quality. Therefore,C is defined to be than 0.02%.

EXAMPLE 9 tons of a molten steel of 0.06% C, 0.14% Mn and 0.010% S madein a converter were bottom-poured into a downwardly expanding flat moldand 0.9 kg./ton of Al was added thereto immediately before thecompletion of the pouring to produce an ingot. This ingot washot-rolled, was then cold-rolled to be 0.8 mm. thick at a reduction rateof 68% and was annealed at 710 C. for 5 hours to obtain a cold-rolledsteel plate of 0.050% C, 0.14% Mn, 0.01% S, 0.038% acid-soluble Al and0.014% P, the rest being Fe and impurities. The surface test result andcoldformability in this case as compared with those in a conventionalmethod are shown in Table 1. The conventional method here designates atreatment of a rim-stabilized ingot containing 0.07% C, 0.31% Mn and0.015% S by the same process as is mentioned above. By the followingcomparison the effect of the method of the present invention can clearlybe demonstrated.

What is claimed is:

1. A process for producing cold-rolled steel plates having excellentcold-formability comprising the steps of pouring into a mold a moltensteel made in a converter or open-hearth furnace and consistingessentially of a composition of C 50.07 Wt. percent, 0.04 to 0.20 wt.percent Mn, 0.004 to 0.020 wt. percent S, the ratio of Mn to S being atleast 7, not more than 0.0030% N, and the balance iron and impurities;allowing said molten steel to rim for a time interval in said mold; thenkilling the core of said molten steel by adding Al to the molten steelafter said time interval, so that the acid-soluble AI in the remainingmolten steel may be more than 0.010% to obtain an ingot; and subjectingthe ingot to hot-rolling, cold-rolling, recrystallizing and annealing.

References Cited UNITED STATES PATENTS 3,239,390 3/1966 Matsukura et al.14812 3,496,032 2/1970 Shimizu et al. l4812 RICHARD O. DEAN, PrimaryExaminer U.S. Cl. X.R.

